Background Oncological sigmoid and rectal resections are accompanied with substantial risk of anastomotic leakage. Preoperative risk assessment and patient selection remain difficult, highlighting the importance of finding easy‐to‐use parameters. This study evaluates the prognostic value of contrast‐enhanced (CE) computed tomography (CT)‐based muscle measurements for predicting anastomotic leakage. Methods Patients that underwent oncological sigmoid and rectal resections in the LUMC between 2016 and 2020 were included. Preoperative CE‐CT scans, were analyzed using Vitrea software to measure total abdominal muscle area (TAMA) and total psoas area (TPA). Muscle areas were standardized using patient's height into: psoas muscle index (PMI) and skeletal muscle index (SMI) (cm2/m2). Results In total 46 patients were included, of which 13 (8.9%) suffered from anastomotic leakage. Patients with anastomotic leakage had a significantly lower PMI (22.1 vs. 25.1, p < 0.01) and SMI (41.8 vs. 46.6, p < 0.01). After adjusting for confounders (age and comorbidity), lower PMI (odds ratio [OR]: 0.85, 95% confidence interval [CI] 0.71–0.99, p = 0.03) and SMI (OR: 0.93, 95%CI 0.86–0.99, p = 0.02) were both associated with anastomotic leakage. Conclusion This study showed that lower PMI and SMI were associated with anastomotic leakage. These results indicate that preoperative CT‐based muscle measurements can be used as prognostic factor for risk stratification for anastomotic leakage.